This invention relates broadly to procedures for effecting sand control in a sub-surface earth formation. More specifically, the invention concerns an apparatus useful for testing a sample of unconsolidated sand for the purpose of simulating actual consolidation of the sand body in the formation.
One of the problems encountered in obtaining oil or gas from a subsurface earth formation is unconsolidated sand, which can be carried into the well bore along with the oil or gas. The problem can be alleviated by treating the formation sand with various chemicals to consolidate the sand particles. Before actual consolidation treatment is begun, a sample of the unconsolidated sand is tested for fluid permeability and other characteristics. The sample is then treated with the appropriate chemicals under simulated formation conditions.
A common procedure for the simulated test involves placing a container, such as a metal cylinder which is closed at one end, on a vibrating table. With the table vibrating, 100cc of water (approximately 35% of capacity) is placed in the cylinder, and the unconsolidated sand is sifted into the cylinder until it is full of moist, compacted sand. The open end of the cylinder is then capped and the sample is checked for fluid permeability, using a light oil at residual water saturation. The sand sample is then treated to consolidate the particles and the permeability of the sample to diesel oil is determined.
The prior test procedure has several disadvantages. One drawback is that many of the unconsolidated soils to be tested contain high concentrations of silt, clay, or other non-sand fractions. When sand of this type is packed into the test container, the difference in texture of the sand and non-sand fractions causes a segregation within the sample. The result is that a uniform, isotropic sand pack cannot be obtained. In a sand pack of a non-uniform character, it is difficult to obtain an accurate flow profile in checking the sample for fluid permeability. Also, the sand pack will have low fluid permeability, which would require high injection pressures. When a treating fluid is forced through a low permeability sample, therefore, such as a sample containing non-sand fractions, the high pressure required to force the fluid through the sand pack will disturb the pack. This disturbance will alter the flow profile of the pack and thus result in incomplete coverage of the compacted sample by the treating fluids.
Another disadvantage of the vibration technique is that the amount of compaction which can be imparted to each sample is not always reproducible from one test to another. For example, reproducibility of compaction for sand which is clean of silt and clay is good. In a sand sample which contains silt and clay fractions (dirty sands), however, the compaction reproducibility is very poor. In fact, the compaction reproducibility becomes worse as the sand becomes dirtier. Since this test also requires compaction reproducibility for dirty sands, in order to get an accurate flow profile in the compacted sample, the objective was to find a way to solve the problem. The sampling apparatus of this invention achieves this objective.
Another reason which makes it difficult to obtain a uniform sand pack using the prior procedure is a natural occurrence known as the arching principle. An illustration of the arching principle is that when particulate matter, such as sand, is placed in a cylindrical vessel the particle mass will arch itself to the walls of the container if the ratio of the container diameter to the length is more than 1 to 1. As a specific example, if the ratio is about 1 to 3, or higher, several arched areas will be present in the compacted sample. Where the sample has distinct arched areas therefore, the porosity of the sample will vary enough so that there is a definite non-uniformity in the sand pack.